1. Field of the Invention
The present invention relates to a voltage measuring device for measuring a voltage across each of unit cells connected in series and composing an assembled battery.
2. Description of the related art
These days, a hybrid electric vehicle (hereunder referred to as “HEV”) using an electric motor together with an engine becomes widely used. This HEV includes a low voltage battery such as 12 V for starting the engine, and a high voltage battery for driving the electric motor. The high voltage battery obtains a high voltage by a plurality of unit cells connecting in series. The unit cell is a secondary battery such as a nickel hydride battery or a lithium battery.
The voltage across each unit cell, namely, a state of charge (SOC) of the high voltage battery is varied while repeating charge and discharge. In the event of charge and discharge of the battery, from a viewpoint of durability of each unit cell and a viewpoint of security, it is necessary to restrict the charge when the unit cell having the highest SOC reaches an upper limit SOC (or upper limit voltage), and to restrict the discharge when the unit cell having the lowest SOC reaches a lower limit SOC (or lower limit voltage).
Accordingly, when the unit cells are varied, substantially, a usable capacity of the battery is reduced. Therefore, in the HEV, so called assisting-regenerating system works insufficient, so that a vehicle performance and a mileage are reduced. According to the assisting-regenerating system, the electric motor using the battery assists the engine when a vehicle goes up a hill, and the battery is charged when the vehicle goes down the hill. Accordingly, for equalizing the SOCs of the unit cells, measuring the voltage across each unit cell is needed.
Conventionally, a device shown in FIG. 10 is disclosed as a voltage measuring device 10 for measuring a voltage across each unit cell composing a high voltage battery (for example, see Patent Document 1). In FIG. 10, a reference mark BH indicates a high voltage battery. The high voltage battery BH is used as a power source for an electric motor M of the HEV. The electric motor M is connected to both ends of the high voltage battery BH as needed. Further, an alternator (not shown) as a battery charger is connected to the both ends of the high voltage battery BH as needed. The high voltage battery BH is composed of, for example, five unit cells C1 to C5 each of which is a secondary battery.
The voltage measuring device 10 includes a capacitor C sequentially connected to the unit cells C1 to C5, and a plurality of selector switches S1 to S16 for sequentially connecting the unit cells C1 to C5 to both ends of the capacitor C.
The six selector switches S11 to S16 are provided against the five unit cells C11 to C5. Namely, for example, an anode of the unit cell C1 and a cathode of the unit cell C2 connected to the anode of the unit cell C1 are connected to the capacitor C via a common selector switch S12. As shown in FIG. 10, using the common selector switches S12 to S15 reduces the number of the selector switches.
The voltage measuring device 10 also includes a voltage measuring circuit 11 for measuring a voltage across the capacitor C, and a pair of switches S21 and S22 respectively provided between one end of the capacitor C and an input terminal T1 of the voltage measuring circuit 11, and between the other end of the capacitor C and an input terminal T2 of the voltage measuring circuit 11. The voltage measuring circuit 11 works with a low voltage battery (not shown) electrically isolated from the high voltage battery BH. The voltage measuring circuit 11 is composed of a differential amplifier 11a for outputting a difference between voltages inputted from the input terminals T1 and T2, and an A/D converter 11b for converting an analog output from the differential amplifier 11a to a digital signal.
An operation of the voltage measuring device 10 will be explained below. Switches S11 to S16, S21, and S22 are normally off. From this condition, when the selector switches S11 and S12 are on, a voltage across the unit cell 1 is charged to the capacitor C. Next, after the selector switches S11 and S12 are off, the measuring switches S21 and S22 are on, then the both ends of the capacitor C are connected to the input terminals T1 and T2 of the voltage measuring circuit 11.
Then, the differential amplifier 11a in the voltage measuring circuit 11 supplies a voltage across the capacitor C to the A/D converter 11b. The A/D converter 11b converts the voltage across the capacitor C to measure the voltage as a voltage across the unit cell C1. Similarly, the selector switches S12 to S16 are sequentially on, and the voltages across the unit cells C2 to C5 are sequentially measured. Because the selector switches S11 to S16 and the measuring switches S21 and S22 are not simultaneously on, the high voltage battery BH is electrically isolated from the voltage measuring circuit 11 while measuring the voltage.
Incidentally, in the voltage measuring device 10 shown in FIG. 10, when the unit cells having odd numbers C1, C3, C5 is connected to the capacitor C, an one terminal “a” of the capacitor is plus charged, and the other terminal “b” is minus charged. On the other hand, when the unit cells having even numbers C2, C4 are connected to the capacitor C, the other terminal “b” is plus charged, and the one terminal “a” is minus charged. Accordingly, the capacitor C is inversely charged depending on the connected unit cells C1 to C5.
Therefore, conventionally, for measuring the voltage across the capacitor charged inversely, a bipolar input A/D converter 11b is used, and a polarity output of the A/D converter 11b is ignored. Alternatively, an absolute value circuit is provided between the differential amplifier 11a and the A/D converter 11b. However, in the conventional voltage measuring device 10, a complex and expensive component such as the bipolar input A/D converter or the absolute value circuit is required, therefore the conventional voltage measuring device 10 costs high.
Therefore, as shown in FIG. 11, reversing switches S23 and S24 are added to the measuring switches S21 and S22. Thus, for always connecting an anode of the capacitor C to the input terminal T1, and always connecting a cathode of the capacitor C to the input terminal T2, the measuring switches S21, S22 and the reversing switches S23, 24 are on/off controlled.
In the voltage measuring device 10, an upper level cell is charged higher voltage. For example, when each of unit cells C1 to C5 is charged 12V, the voltage of the anode of the uppermost cell C1 is 60V (=12V*5), and the voltage of the cathode of the uppermost cell C1 is 48V (=12V*4). Accordingly, when the unit cell C1 charges the capacitor C, the voltage of the terminal “a” is 60V, and the voltage of the terminal “b” is 48V. Resultingly, 60V and 48V are respectively applied to the input terminals T1, T2 of the differential amplifier 11a. Therefore, high voltage switches for the high voltage battery BH should be used as the measuring switches S221, S22, and the reversing switches S23, S24, and the voltage measuring device 10 costs high.
Further, there is a problem that a break between the unit cells C1 to C5 and the capacitor C cannot be detected. For example, in FIG. 10, suppose that a break is occurred at T3. In that case, firstly, the unit cell C1 charges the capacitor C, next, the unit cell C2 charges the capacitor C. However, because the T3 is broken, the unit cell C2 cannot charge the capacitor C, and the voltage across the capacitor is a residual voltage charged by the unit cell C1.
For solving the problem, for example, according to the Patent Document 2, a series circuit composed of a discharge resistor and a reset switch parallel to the capacitor C is provided, and at every end of measuring the voltage across the capacitor C, the reset switch is on to discharge the capacitor C. By discharging the capacitor C at every measuring, if the break is occurred at T3, the voltage across the capacitor C after charged by the unit cell C2 is zero or a very small value, so that the break can be detected.
However, according to a method of the Patent Document 2, the reset switch is needed in addition to the switch for connecting the both terminals of the capacitor C and the voltage measuring circuit 11. Therefore, the voltage measuring device 10 costs high.
[Patent Document 1] Japanese Published Patent Application No. H11-248755
[Patent Document 2] Japanese Published Patent Application No. 2003-84015
According to the above, an object of the present invention is to provide a low-cost voltage measuring device for preventing a negative voltage from applying to a voltage measuring unit when a capacitor charged in positive and negative polarities depending on unit cells to be connected to the capacitor is connected to the voltage measuring unit, and for allowing a switch which is not a high voltage switch to be used as a switch between the capacitor and the voltage measuring unit.